Piezoresistive materials experience a change in one or more electrical properties of the material when subjected to a strain. For example, the effect of strain on silicon (Si) and germanium (Ge) was discovered in the 1950s. It was found that strain on these materials can result in the modification of several electrical properties of the materials such as band gap, carrier mobility and so on. As a result, the resistance or otherwise resistivity of the material is also altered because of the strain. This effect is known as piezoresistivity or piezoresistance (PZR) effect.
This property of piezoresistive materials has found widespread applications in Micro Electro Mechanical Systems (MEMS) sensor systems, particularly as displacement sensors and force sensors. Even small forces on such PZR sensors can produce strain and thereby a measurable change in the resistance or otherwise resistivity of such devices. This change in resistance or otherwise resistivity can be correlated to the displacement of the piezoresistive material and/or the amount of force exerted on the piezoresistive material. Another application involves intentionally applying strain on transistors to enhance carrier mobility and improve the transistor's performance.
The most common material used for fabricating PZR devices includes Si. State-of-the-art PZR devices include Si nanowires which are generally manufactured using a chemical vapor deposition (CVD) process. A PZR coefficient of such silicon nanowires is higher than bulk silicon by a factor of about 2. Although such Si nanowires have shown great promise in sensing applications (e.g., force sensing, and bio sensing applications) which use the PZR effect of the Si nanowires for sensing, strong and rapid oxidation of the Si nanowires impacts the performance of such sensors. Oxidation of the Si nanowires can change the mechanical properties (e.g., moment of inertia, strain, etc.) and/or electrical properties (e.g., resistivity) of the Si nanowires thereby affecting measurements. Furthermore, the Si nanowires are known to lose their functionality at temperatures higher than about 300 degrees Celsius, which has limited the widespread use of such Si nanowires as force sensors.